1. Field of the Invention
The invention relates to image processing method and apparatus for inputting digital image data and converting to image data whose number of bits is smaller than that of the input data and, more particularly, to image processing method and apparatus for enabling a half tone image to be displayed for a display using, for example, a ferroelectric liquid crystal and having a gradation expressing ability of a small number of gradations such as two values, four values, or the like.
2. Related Background Art
A ferroelectric liquid crystal (hereinafter, simply referred to as an FLC) display among liquid crystal displays has a feature such that the liquid crystal has a "memory performance" to hold a display state changed by applying an electric field, so that a contrast is not deteriorated even if the number of scan lines increases. Therefore, an attention is paid to the FLC display as a device which can display by a large screen and at a high precision even in a simple matrix structure.
However, the FLC fundamentally can express only two gradations by one element. Even in case of displaying one pixel by a plurality of elements, the number of gradations which can be expressed is four values, six values, eight values, or the like and its gradation expressing ability is low.
Hitherto, a half tone process such as dither method, error diffusion method, or the like has been used as a method of improving the gradation expressing ability of a display device of a low gradation expressing ability. Such half tone processing methods have already widely been used in a printer apparatus of a binary output and the like.
According to the dither method, although an output value is decided by comparing an input value and a predetermined threshold value, by changing the threshold value in a predetermined area, a half tone is expressed in the predetermined area in a macro manner. As an example of a multivalue dither in which the dither process is executed to a device of a multivalue output, a case where a 4-level dither process by a (2.times.2) dither matrix is executed to input data of eight bits and a 4-level output of 0 to 3 is obtained will now be described with reference to the drawings. In case of the 4-level dither of the (2.times.2) dither matrix, four threshold tables each having three threshold values as shown in FIG. 2 are used in correspondence to cells; of a dither matrix as shown in FIG. 3. The dither matrix is moved and applied to each pixel in the image data at a period of two pixels in both of the vertical and horizontal directions. The corresponding pixel data is compared with a proper one of the threshold tables, and either one of the four values which can be outputted by the device is selected and outputted. For example, assuming that the input data is 100 and the pixel position corresponds to the position of the dither matrix 1 in FIG. 3, a threshold table 1 is applied. In this case, since 100 is larger than the second small threshold value 95 in a threshold table 1, an output value is set to 2. However, in the case where the pixel position corresponds to the position of 2 of the dither matrix, a threshold table 2 is selected. Even when the input value is equal to 100, since it is smaller than a threshold value 117, the output value is set to 1. In the case where the pixel positions correspond to the positions of 3 and 4 of the dither matrix, the output values are also set to 1. In such an area of (2.times.2), a combination of outputs of 2/1/1/1 is obtained and a half tone of 1.25 is expressed as a whole matrix. By also executing similar processes to the other input values, a correspondence relation of the input value/output value as shown in FIG. 2 is obtained every pixel position. A half tone can be falsely obtained as a combination of them.
However, when the image which was dither processed is continuously displayed on the display or the like as a moving image, in case of a system construction such that noises are mixed to the input data, there is a situation such that the input data changes while fluctuating near the threshold value due to the noises in spite of the fact that the input value is inherently equal.
For example, in FIG. 2, in the case where although the input data at the pixel position to be compared with the threshold table 1 is equal to 94 at the Nth frame, it changes from 94 to 96 by the noises at the (N+1)th frame, the output value changes from 1 to 2. Further, when the input data is returned to 94 at the (N+2)th frame, the output value also changes from 2 to 1. Therefore, even when the fluctuation of the input value is small, such a fluctuation is magnified and appears in a change in output. When such a phenomenon occurs in the whole screen, a flickering phenomenon of the whole screen, namely, so called scintillation noises occur. A display quality of a moving image is remarkably deteriorated. There is a drawback such that an occurrence frequency of the above problem is likely to increase as the number of gradations of the output value is large, namely, as the number of threshold values is large.